Is there a first-order discontinuity in the lowermost mantle?

Liu, Xian Feng; Tromp, Jeroen; Dziewoński, Adam M.

doi:10.1016/s0012-821x(98)00095-8

Cited by 17 publications

(12 citation statements)

References 26 publications

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“…This is only marginally consistent with the observed seismic velocity variations [Weber, 1993;Lay et al, 1997;Liu et al, 1998;Castle et al, 2000]. Other factors such as chemical heterogeneities and anisotropy can be more important for seismic velocity variations.…”

Section: Lateral Temperature Variationsmentioning

confidence: 46%

“…Lateral heterogeneities in the D 00 layer are present on scales as small as 100 -300 km. These are associated either with topography of the D 00 discontinuity or lateral variations in seismic velocities in the D 00 layer [Weber, 1993;Lay et al, 1997;Liu et al, 1998]. Anisotropy has also been established in many regions of the D 00 layer [Lay et al, 1998b;Kendall and Silver, 1996;Matzel et al, 1996;Garnero and Lay, 1997].…”

Section: Introductionmentioning

confidence: 99%

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Small‐scale convection in the D″ layer

Solomatov

Moresi

2002

J. Geophys. Res.

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[1] Small-scale convection has been suggested as a possible explanation for seismic heterogeneities in the D 00 layer of the Earth's mantle. Recently developed scaling laws for convection with realistic viscosities allow quantitative assessment of this hypothesis. Large temperature and viscosity contrasts across the thermal boundary layer at the core-mantle boundary suggest that small-scale convection starts at the bottom of the thermal boundary layer and propagates upward before the thermal boundary layer as a whole becomes unstable. The convection boundary is likely to become a chemical boundary as a result of mixing within the convective layer. This implies that the D 00 discontinuity can represent both convective and chemical boundary and that the presence or absence of small-scale convection can be responsible for the observed intermittent nature of the D 00 discontinuity. Most lateral heterogeneities in the D 00 layer are concentrated near the convection boundary, consistent with seismic data. The length scale of lateral temperature variations, the topography of the core-mantle boundary, and the viscosity of the D 00 layer are in agreement with observational constraints. The thickness of the secondary thermal boundary layer formed at the bottom of the convective layer is similar to the thickness of the ultralow-velocity zone. The magnitude of lateral variations in temperature can only marginally be responsible for lateral variations in seismic velocities. Variations in the topography of the convection boundary and chemical heterogeneities are likely to be more important factors. A large seismic velocity drop in the ultralow-velocity zone cannot be explained by thermal effects alone and must be caused by other factors such as partial melting.

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Section: Lateral Temperature Variationsmentioning

confidence: 46%

Section: Introductionmentioning

confidence: 99%

Small‐scale convection in the D″ layer

Solomatov

Moresi

2002

J. Geophys. Res.

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“…The fact that we observe no evidence for a P wave reflector where there is evidence for a $ wave reflector suggests a chemical (at least partially) origin of the P* and $* phases. Liu et al [1998] propose that these phases are due to strong velocity gradients rather than first order discontinuities [see also Tromp and Dziewonski, 1998]. In this case, P* phases are less likely to exist than $* because P wave velocity variations are smaller than $ wave velocity variations.…”

Section: Error Estimationmentioning

confidence: 99%

A waveform migration for the investigation of P wave structure at the top of D″ beneath northern Siberia

Freybourger¹,

Chevrot²,

Achauer³

2001

J. Geophys. Res.

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Abstract. We present the first waveform migration for the lower mantle, focusing on the lowermost 400 km of the mantle beneath northern Siberia. We migrate vertical component ? wave seismograms recorded in western Europe for earthquakes located in the Kuril Islands region. We focus on the time window between the direct P wave and the ?c? wave, reflected at the core-mantle boundary, and we investigate the origin of precursors. The data, selected for their good signal-to-noise ratio, are deconvolved by the source time function determined by the beam of the P wave. We observe the signature of a finite size scattering object located around 75øN, 85øE, at an apparent depth of •2650 km with respect to the IASP91 model. Its scale length is probably of the order of 100 km. This object is characterized by an anisotropic scattering diagram and may be related to the presence of an ancient subducted plate near the core-mantle boundary.

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“…The reason we chose an elliptical geometiy to desclibe the anomalies was the shape of the tomographic fast anornaly and the simplicity of the calc~1lations. So far velocity heterogeneities of only several percent with lateral scale lengths of several h~mdred hlometers in Earth's mid-mantle and lowellnost mantle have been calculated with tomographic rnodels (16,17,19,20). Recently it was fo~lnd that anomalies in the lowermost -100 lulometers of the lnantle (21-24) co~lld be much larger (reductions of 10% or more in P and up to 30% reductions in S velocities).…”

Section: Lower Mantle Lateral Heterogeneity Beneath the Caribbean Seamentioning

confidence: 99%

“…Recently it was fo~lnd that anomalies in the lowermost -100 lulometers of the lnantle (21-24) co~lld be much larger (reductions of 10% or more in P and up to 30% reductions in S velocities). One of the problems of the existing global tomographic rnodels (19)(20)(21) is that they are the res~llt of damped least squares inversions that underestimate the anolnalous velocity gradients and overestimate the width of the heterogeneity. It seems that larger velocity anomalies are possible in confined zones that affect the travel time of P waves (by as much as 1.4 s) and cause the waves to be deflected fiom their normal path.…”

Section: Lower Mantle Lateral Heterogeneity Beneath the Caribbean Seamentioning

confidence: 99%

Lower Mantle Lateral Heterogeneity Beneath the Caribbean Sea

Tibuleac

Herrin

1999

Science

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Seismic wave reflections from Earth's core recorded at seismic arrays in North America from events in the Caribbean Islands, Venezuela, and the Mid-Atlantic Ridge have observed slownesses more than 64 percent greater than predicted by the IASPEI91 standard Earth model. P waves turning in the lowermost mantle beneath the same region also have anomalous slowness. The slowness anomalies are not accompanied by significant travel time residuals and appear to be caused by lateral inhomogeneities in the velocity structure of the lower mantle.

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Is there a first-order discontinuity in the lowermost mantle?

Cited by 17 publications

References 26 publications

Small‐scale convection in the D″ layer

Small‐scale convection in the D″ layer

A waveform migration for the investigation of P wave structure at the top of D″ beneath northern Siberia

Lower Mantle Lateral Heterogeneity Beneath the Caribbean Sea

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